1. Field of the Invention
The present invention relates to a communication technology for a mobile communication terminal that uses an adaptive array antenna.
2. Related Art
In recent years, increasing attention has been given to the adaptive array method in the field of mobile communication to make efficient use of frequencies and improve communication quality. The adaptive array method is a communication method that performs transmission/reception by adjusting a signal amplitude and phase for each of a plurality of antennas, thereby enhancing a radiant intensity or reception sensitivity with regard to a direction from which a desired signal comes.
A mobile communication system that uses the adaptive array method employs a communication technology in which a base station, equipped with an adaptive antenna array made up of multiple antennas and a control unit, tracks the movement of a mobile communication terminal.
Also, with the recent development of small and light antennas such as a tip antenna, it becomes possible to integrate an adaptive antenna array into a mobile communication terminal.
The following describes how a mobile communication terminal which includes adaptive array antennas controls directivity patterns.
For example, a PHS (personal Handyphone System), a mobile communication terminal, performs transmission and reception alternately in a TDD frame unit using a TDD (Time Division Duplex) method. When receiving a frame, the mobile communication terminal receives known signals such as preambles and unique words from a base station via a plurality of antennas. The mobile communication terminal adds weights to the received signals for each antenna using weight vectors and combines the weighted signals. The weight vectors are adjusted so as to minimize the difference between the resulting combination of signals and a reference signal that is stored in advance (a signal indicating a preamble or a unique word) for the purpose of optimizing the weight vectors. Using the optimum weight vectors, each signal received via the plurality of antennas is weighted and the weighted signals are combined, so that the mobile communication terminal can receive desired signals from the base station. When transmitting a signal to the base station, the mobile communication terminal distributes transmission signals to the plurality of antennas and applies weights to the distributed signals using the weight vectors calculated at the time of reception. Here, to perform reception or transmission with a reception sensitivity or radiant intensity being increased in a predetermined direction is referred to as forming a directivity pattern and performing transmission or reception.
However, due to problems such as weak electrical fields, interference and losses of synchronization, there are cases where the mobile communication terminal cannot receive signals properly from the base station. In this case, transmission signals in the same frame will not be able to reach the base station with which the mobile communication terminal is communicating.
Which is to say, when the mobile communication terminal fails to properly receive signals from the base station, it calculates a weight vector from the improperly received signals and generates a directivity pattern. Because such a pattern does not have directivity towards the intended base station, transmission signals will not reach the base station. This also makes it impossible for the base station to generate a directivity pattern that has directivity towards the mobile communication terminal. Since neither the mobile communication terminal nor the base station can form directivity patterns towards the other device, their transmission signals will not reach the destination. Unless the channel in use is switched, normal communication between them will not be restored.
In addition, the use of a wrong weight vector causes the formation of a directivity pattern having increased radiant intensity towards another unrelated base station or mobile communication terminal, thereby interfering with such unrelated base stations and mobile communication terminals.